Thank you both!
sslyutov, your description is exactly what I see in another PC. But in this PC, there is only the "unknown device".
And one more thing. I remember that, initially, this PC could recognize the Casio MIDI when I first installed windows 10. But after using it for a while and also having some other software installed, the problem arose. So, I'm not yet sure whether it's a compatibility issue or firmware issue.